The invention concerns suppression of radio frequency interference produced by a DC motor in which speed is controlled by Pulse Width Modulation (PWM).
FIG. 1 illustrates a motor M used in the prior art, such as to power a blower in an automotive air conditioner. A vehicle battery BAT powers the motor. The motor is contained within a conductive housing H. The motor contains brushes and a commutator (not separately shown) which cause electrical arcing and sparking, and produce high-frequency Radio Frequency Interference (RFI).
The RFI can interfere with communication devices, such as radios, tape players, televisions, ham radios, and cellular telephones. Measures taken to reduce the RFI include connecting the housing H to the negative terminal, B-, of the motor, as indicated by wire W, and installation of a filter which includes inductors L1, L2, and capacitor C1.
The motor shown uses a simple type of speed control, illustrated by the adjustable resistor R. Adjusting the resistor modifies the voltage presented to motor terminals B+ and B-, by the voltage divider principle However, such speed controls, although simple, are inefficient, because power is dissipated in the resistor. Not only does this dissipation represent wasted power, but the wasted power is dissipated as heat, which poses a heat elimination problem.
To avoid these problems, the inventors have investigated Pulse Width Modulation (PWM) for controlling motor speed.